In recent years, composite materials reinforced with fibers, particularly carbon fibers, have been widely used in the production of sporting goods, such as shafts of golf clubs and fishing rods. In many cases, these composite materials are used in the form of prepregs. As a matrix resin to be used in the production of prepregs, an epoxy resin is mainly used, from the viewpoint of increasing the mechanical strength.
It is known that in such epoxy resin compositions, dicyandiamide is used as a hardening agent, and urea derivatives such as dichlorophenyl-1,1-dimethylurea are used as hardening-accelerating agents, as described in U.S. Pat. No. 4,500,660. These epoxy resin compositions have a shelf life of more than one month at room temperature and are hardenable at 130.degree. C. Thus, they are now widely used.
As low temperature hardening agents, i.e., hardening agents which allow the epoxy resin composition to polymerize at temperatures ranging between room temperature and 100.degree. C., imidazole compounds such as 2-ethyl-4-methylimidazole and polyamines are known.
In addition, as hydrazide-based hardening agents, adipic acid dihydrazide (m.p., 180.degree. C.), sebacic acid dihydrazide (m.p., 187.degree. C.), isophthalic acid dihydrazide (m.p., 219.degree. C.), malonic acid dihydrazide (m.p., 152.degree.-154.degree. C.), oxalic acid dihydrazide (m.p., 241.degree. C.), succinic acid dihydrazide (m.p., 167.degree.-168.degree. C.), and the like are known, as described, for example, in Japanese Patent Application (OPI) No. 25700/75. (The term "OPI" as used herein refers to a published unexamined Japanese patent application.)
Epoxy resin compositions containing dicyandiamide as a hardening agent and urea derivatives such as dichlorophenyl-1,1-dimethylurea as hardening-accelerating agents, as described above, have a shelf life of more than one month at room temperature and are hardenable at 130.degree. C. However, in cases where these epoxy resin compositions are reinforced with fibers having a small coefficient of thermal expansion, such as carbon fibers, as the molding temperature is increased, the residual stress in the ultimate molded article is increased, thereby causing problems such as cracking.
Epoxy resin compositions containing imidazole compounds such as 2-ethyl-4-methylimidazole and polyamides as hardening agents are hardenable at low temperatures, but have a disadvantage in that their shelf life is short.
Also, epoxy resin compositions using hydrazide-based hardening agents as described above have disadvantages in that temperatures higher than 150.degree. C. are needed for molding and long periods of time are needed for hardening.
In order to overcome the above problems, and furthermore to achieve energy-saving and increase operation efficiency, it has been desired to develop a monoliquid type epoxy resin composition which can be completely hardened at a relatively low temperature and has a long shelf life.
Also, in view of the operation environment and influences exerted on the ultimate product by residual solvents, it has been desired to develop a resin composition which is suitable for use in the production of prepregs by the hot melt method without use of a solvent.